Merge branch 'jk/fetch-ref-prefix-cleanup'

In protocol v2 where the refs advertisement is constrained, we try
to tell the server side not to limit the advertisement when there
is no specific need to, which has been the source of confusion and
recent bugs.  Revamp the logic to simplify.

* jk/fetch-ref-prefix-cleanup:
  fetch: use ref prefix list to skip ls-refs
  fetch: avoid ls-refs only to ask for HEAD symref update
  fetch: stop protecting additions to ref-prefix list
  fetch: ask server to advertise HEAD for config-less fetch
  refspec_ref_prefixes(): clean up refspec_item logic
  t5516: beef up exact-oid ref prefixes test
  t5516: drop NEEDSWORK about v2 reachability behavior
  t5516: prefer "oid" to "sha1" in some test titles
  t5702: fix typo in test name